Virtues are traits that one ought to possess, for the benefit of the world or oneself.

On LessWrong the focus is often on epistemic virtues, as in Eliezer Yudowsky's essay Twelve Virtues of Rationality which offers this list of virtues (roughly summarized):

Curiosity - the burning desire to pursue truth;
Relinquishment - not being attached to mistaken beliefs;
Lightness - updating your beliefs with ease;
Evenness - not privileging particular hypotheses in the pursuit of truth;
Argument - the will to let one's beliefs be challenged;
Empiricism - grounding oneself in observation and prediction;
Simplicity - elimination of unnecessary detail in modeling the world;
Humility - recognition of one's fallibility;
Perfectionism - seeking perfection even if it's not attainable;
Precision - seeking narrower statements and not overcorrect;
Scholarship - the study of multiple domains and perspectives;
The nameless virtue - seeking truth and not the virtues for themselves.

See Also: Courage, Trust, Honesty, Agency, Altruism, Ambition, Stoicism / Letting Go / Making Peace, Attention, Gratitude

Sequences:
Notes On Virtues by David_Gross